1. Field of the Invention
The present invention relates to a power supply apparatus suitable for an image forming apparatus which forms an image by an electrophotographic process and, more particularly, to a power supply apparatus using a piezoelectric transformer and an image forming apparatus using the power supply apparatus.
2. Description of the Related Art
When an image forming apparatus which forms an image by an electrophotographic process adopts a direct transfer system of transferring an image by bringing a transfer member into contact with a photosensitive member, the transfer member uses a conductive rubber roller having a conductive rotating shaft. In this case, driving of the transfer member is controlled to match the process speed of the photosensitive member.
A voltage applied to the transfer member is a DC bias voltage. At this time, the polarity of the DC bias voltage is the same as that of a transfer voltage for general corona discharge. To achieve satisfactory transfer using the transfer roller, a voltage of generally 3 kV or more (the required current is several μA) must be applied to the transfer roller. This high voltage necessary for the above image forming process is conventionally generated using a wire-wound electromagnetic transformer. The electromagnetic transformer is made up of a copper wire, bobbin, and core. When the electromagnetic transformer is used in application of a voltage of 3 kV or more, the leakage current must be minimized at each portion because the output current value is as small as several μA. For this purpose, the windings of the transformer must be molded with an insulator, and the transformer must be made large in comparison with the magnitude of the supply power. This inhibits downsizing and weight reduction of a high-voltage power supply apparatus.
In order to compensate for these drawbacks, generation of a high voltage using a flat, light-weight, high-output piezoelectric transformer is examined. By using a piezoelectric transformer formed from ceramic, the piezoelectric transformer can generate a high voltage at higher efficiency than that of the electromagnetic transformer. Since electrodes on the primary and secondary sides can be spaced apart from each other regardless of coupling between the primary and secondary sides, no special molding is necessary for insulation. The piezoelectric transformer brings an advantage of making a high-voltage generation apparatus compact and lightweight.
For example, Japanese Patent Laid-Open No. 11-206113 discloses a high-voltage generation apparatus using a piezoelectric transformer.
A high-voltage power supply circuit using a piezoelectric transformer will be explained with reference to FIG. 13. In FIG. 13, reference numeral 101Y denotes a piezoelectric transformer (piezoelectric ceramic transformer) for a high-voltage power supply. Diodes 102Y and 103Y and a high-voltage capacitor 104Y rectify and smooth an output from the piezoelectric transformer 101Y to a positive voltage, and a transfer roller (not shown) serving as a load receives it. Resistors 105Y, 106Y, and 107Y divide the output voltage, and the inverting input terminal (negative terminal) of an operational amplifier 109Y receives it via a protection resistor 10Y. The non-inverting input terminal (positive terminal) of the operational amplifier receives, via a resistor 114Y, a high-voltage power supply control signal Vcont which serves as an analog signal and is input to a connection terminal 118Y from a DC controller 201. The operational amplifier 109Y, the resistor 114Y, and a capacitor 113Y construct an integrating circuit. The operational amplifier 109Y receives control signal Vcont smoothed by an integral time constant determined by the component constants of the resistor and capacitor. The output terminal of the operational amplifier 109Y is connected to a voltage-controlled oscillator (VCO) 110Y. A transistor 111Y whose output terminal is connected to an inductor 112Y is driven to supply power to the primary side of the piezoelectric transformer.
The high-voltage power supply unit of an electrophotographic image forming apparatus comprises a plurality of high-voltage power supply circuits using the piezoelectric transformer shown in FIG. 13. The high-voltage power supply unit corresponding to image forming units for, e.g., yellow (Y), magenta (M), cyan (C), and black (BK) outputs biases for charging, development, transfer, and the like to form images.
In the above example, pluralities of piezoelectric transformers and control circuits are arranged in the high-voltage power supply unit, and a plurality of bias voltages are output to form images. Especially, a high-voltage power supply unit mounted in a color image forming apparatus of a tandem system requires four bias output circuits for charging, development, transfer, and the like in correspondence with formation of cyan, magenta, yellow, and black images. The circuits corresponding to cyan (C), magenta (M), yellow (Y), and black (BK) colors are controlled at almost the same bias output voltage. Piezoelectric transformers mounted in the high-voltage power supply unit are driven at almost the same frequency (close frequencies) in the respective bias output circuits (C, M, Y, and BK) for charging, development, transfer, and the like.
A plurality of piezoelectric transformers are driven at close frequencies to output the same bias voltages. In this case, adjacent piezoelectric transformers interfere with each other via the power supply line or depending on electrostatic capacitive coupling or the like, which makes it difficult to improve the output precision of a high bias voltage. Alternatively, the image quality may degrade due to, e.g., generation of fluctuations of a high bias voltage by the interference frequency.
In order to prevent an image from being influenced by the precision of a high bias voltage, piezoelectric transformers are arranged at large intervals. In order to suppress interference via the power supply line, the pattern length is increased or the capacitance of a decoupling capacitor is increased in designing the pattern of the power supply line.
However, it is difficult to analyze these measures by theoretical calculation. Many experiments are required to determine whether the above measures can solve the problem, and concrete measurements must be taken where possible. This prolongs the period of product development. Even when these measures can solve the problem, the high-voltage power supply unit can hardly achieve downsizing and a high image quality at the same time.
The present invention has been proposed to solve the conventional problems, and has as its object to provide a power supply apparatus using piezoelectric transformers which suppresses the interference between the driving frequencies of the piezoelectric transformers, implements downsizing and a high image quality, and requires no experimental measure.
It is another object of the present invention to provide an image forming apparatus having the power supply apparatus.